The present invention relates to compositions which have improved insulation resistance stability and to methods of preparation thereof. More specifically, the invention relates to compositions which have improved insulation resistance stability in water so that compositions can be employed in forming insulation layers which are thinner by a substantial amount than previous insulations and yet provide adequate or improved degrees of insulation resistance stability.
A number of compositions are known which have relatively high insulation resistance stability. Among such compositions are the crosslinked polyethylene compositions prepared generally according to the teachings of the Gilbert and Procopio U.S. Pat. Nos. 2,888,442 and 3,079,370 assigned to the same assignee as the subject application.
It is known that particular properties, such as flame retardancy, electrical properties and moisture resistivities, etc., are of great significance during the actual course of application of polymeric materials in the field. A number of compositions are known which have relatively high flame retardancy properties. Among such compositions are the ones which employ highly effective commercial halogenated flame retardant additives. However, the utilization of halogenated flame retardants in the polymeric materials introduces a high level of polarity into the polymeric materials. The introduction of highly polarized material into the insulating polymeric compositions has long been known to have severely detrimental effects upon the electrical properties, such as insulation resistances and dielectric strength. Such detrimental effects have been accentuated where the polymer compositions containing highly polarized material have been used in wet locations. This is because the presence of the highly polarized ingredient in a polymeric composition has effectively induced absorbtion of moisture. Accordingly, the presence of a highly polarized ingredient in an insulating polymeric composition has precluded long term use of such insulating composition in moist or wet environments. Improvements in the insulation resistivity and resistance to moist environment have been made over a period of time for halogenated compositions. Among such compositions are the ones prepared generally according to the teachings of the Betts And Holub U.S. Pat. No. 4,209,566 assigned to the same assignee as the subject application.
Although such compositions are highly satisfactory and represent an advance in the art, as the needs of the industry change with time, there comes times when still further improvements in the insulation resistance stability of such insulating compositions are desired and needed.
Recently, in particular, there has been a need for providing wire and cable with reduced volume of insulation as the cost of the polymer ingredients have greatly increased. Also, as equipment in which wire has been employed has grown smaller, the need for reducing the wire volume in such equipment has increased.
For example, in a control cable, if a fire insulation thickness is reduced from 30 mils to 20 mils, then the wire diameter is actually reduced by 20 mils due to the 10 mil reduction on each insulating sidewall of the wire. Where a number of such wires are grouped in a control cable, the 20 mil diameter reductions are added for each wire across one diameter of the cable. For example, where seven wires are bundled in a control cable there are essentially three wires across one diameter of the cable, one wire at the center and six wires grouped about the single center wire. For such as cable, the reduction in diameter is about 60 mils for the diameter of the wire bundle. In addition, an outer jacket will have a smaller diameter because of the smaller diameter bundle which it contains.